△ADC的面积为3-√3,则△ADB的面积为(3-√3)/2(因为BD=(1/2)DC)
△ABC的面积为(9-3√3)/2
acsin120=2S△ABC得a=(6√3-6)/c==>BD=(2√3-2)/c
Cos120=(c^2+BD^2-AD^2)/2c*BD
1/2=(c^2+[(2√3-2)/c]^2-4)/2c*[(2√3-2)/c]
化简得c^4-(6-2√3)c^2+16-8√3=0
判别式=2√(2√3-4)
=2√[-(4-2√3)]
=2√[-(√3-1)^2]
[-(√3-1)^2]